Observational foundation for and scaling limitation to sequence modeling

byHarold R. Wanless
Rosenstiel School of Marine and Atmospheric Science, University of Miami

Observational foundation

The design of useful models for predicting sequences and facies patterns of sedimentary cycles depends on an adequate observational foundation. Six facets must be met: 1) adequate documentation of sequence character; 2) characterization of spatial and directional variability and continuity of facies; 3) adequate data base on influence of primary and secondary controls on sedimentation; 4) understanding interaction of controls on sedimentation; 5) understanding limits of physical, biogenic, and chemical influences; and 6) understanding early- to late-stage diagenetic modifications to sequence nature and thickness. Examples from each category illustrate the necessity for meeting each facet.

Spatial and directional variability includes intracycle and intercycle variations in facies type, morphology, and growth habit with orientation of platform margin or coastline; intercycle variations in lateral facies shifts with variations in orientation; variations in early diagenesis with orientation; and variations in emergent topography with orientation.

Primary controls include a) sea-level dynamics; b) sediment supply; c) topography and physiographic setting; d) relative importance of prevailing energy, winter storms, hurricanes, ocean swells, and tides; e) tectonic pulses, flexures, and hinges; and f) climate (especially rainfall and temperature). These primary controls define a variety of secondary controls including organism communities, biogenic sediment production, and water circulation and renewal. Lack of proper consideration of any one primary control can negate the validity of resulting models. In this regard, it is unwise to apply recognized controls on clastic sedimentation to carbonates and vice versa. Clastic and carbonate processes are commonly out of phase with respect to sea level.

Interaction of primary controls on sedimentation commonly generates new processes and facies characteristics that would not be formed independently. For example, sediment accumulation/loss rates and sediment texture are defined by the interaction of a) rate of sediment dispersal by events and platform width; b) potential sediment circulation and windward and leeward margin openness; c) rate of sediment production/supply and shallow-water bioerosion (loss of coarse) or dissolution (loss of fines); d) potential sediment transport and stabilization by vegetation or cementation; and e) surficial sediment accumulation and subsurface replacement by sediment infills of burrow excavations.

Physical, biogenic, and chemical controls have discrete but subtle temporal and spatial boundaries to their influence. For example, winter storms, hurricanes, and other physical controls occur only over certain geographic ranges, and this range changes with geologic time. Subtle changes in water chemistry, temperature, and renewal time define limits for various types of carbonate sedimentation and diagenesis (e.g., ooid production, bioerosion, intragranular and intergranular cementation).

Diagenetic modifications include syndepositional biogenic facies modification through repetitive burrow excavations and infillings causing a) transformation to new sediment composition and permeability in old sediment-body geometries, b) loss of transgressive facies, c) misrepresentation of timing and rates of sedimentation, and d) amalgamation of sedimentary cycles, and later stage compaction and pressure dissolution reducing depositional thickness of sequences.

Scaling

One must be careful in drawing on influences, patterns, and rates from one scale of sedimentation for application to another. Many fundamental controls on sedimentation win have very different effects on different scales of sedimentary cycles. The finer scales of sedimentation may not be conducive to predictive modeling, especially if the driving forces are episodic.

Holocene analogues represent sedimentary cycles in various stages of incompleteness. Integration with Pleistocene sequences are a first step towards assessing the duration necessary for generation of complete cycles and assessing usefulness of Holocene/Pleistocene as analogues to times of dampened eustatic driving forces.